Crystal reports pie chart

I have a record with 2 fields: a date field called 'mydate' and a text field called 'yesno'.
There are about 100 records in the database.
I want to produce a pie chart that will only produce the amount of 'YES' values in the 'yesno' field and show these values on the pie chart by month.
Im pretty new to crystal reports, so if you could give me a step-by-step, that would be great.

I don't quite understand. Is the detail amount correct, just displayed as percentages, or is the actual percentage being displayed.
If the detail amount is being displayed as a percentage, under Chart Expert -> Options -> Number Format is where you can format the field.
If it's the actual percentage being displayed, you're going to have to change the Show Value you're basing the chart on.
I hope this helps.